In the conventional art, in order that communication should be performed between specified persons, a method is adopted in which key information for coding/decoding is shared in transmitting and receiving and in which on the basis of the key information, mathematical arithmetic operation and inverse operation are performed on the information data (plain text) to be transmitted so that concealed communication is achieved FIG. 65 is a block diagram showing a configuration of a conventional data transmitting apparatus according to this method. In FIG. 65, the conventional data communication system has a configuration that a data transmitting apparatus 90001 is connected to a data receiving apparatus 90002 via a transmission path 913. The data transmitting apparatus 90001 comprises an encoding part 911 and a modulating part 912. The data receiving apparatus 90002 comprises a demodulating part 914 and a decoding part 915. In the conventional data communication system, when information data 90 and first key information 91 are inputted to the encoding part 911 while second key information 96 is inputted to the decoding part 915, information data 98 is outputted from the decoding part 915. The operation of the conventional data communication system is described below with reference to FIG. 65.
In the data transmitting apparatus 90001, the encoding part 911 encodes information data 90 (encryption) on the basis of the first key information 91. The modulating part 912 modulates in a predetermined modulation form the information data encoded by the encoding part 911, and transmits as a modulated signal 94 to the data receiving apparatus 90002 via the transmission path 913. In the data receiving apparatus 90002, the demodulating part 914 demodulates by a predetermined demodulation method the modulated signal 94 transmitted via the transmission path 913, and outputs it. The decoding part 915 decodes the signal demodulated by the demodulating part 914 (decryption) on the basis of the second key information 96 shared with the encoding part 911, and reproduces the original information data 98.
A wiretapping action by a third person is described below with reference to a wiretapper data receiving apparatus 90003. In FIG. 65, the wiretapper data receiving apparatus 90003 comprises a wiretapper demodulating part 916 and a wiretapper decoding part 917. The wiretapper demodulating part 916 wiretaps the modulated signal (information data) transmitted between the data transmitting apparatus 90001 and the data receiving apparatus 90002, and demodulates by a predetermined demodulation method the wiretapped modulated signal. On the basis of third key information 99, the wiretapper decoding part 917 tries decoding of the signal demodulated by the wiretapper demodulating part 916. Here, since the wiretapper decoding part 917 does not share the key information with the encoding parts 911, the decoding of the signal demodulated by the wiretapper demodulating part 916 is tried on the basis of the third key information 99 different from the first key information 91. Thus, the wiretapper decoding part 917 cannot correctly decode the signal demodulated by the wiretapper demodulating part 916, and cannot reproduce the original information data.
Such a mathematical encryption technique based on mathematical arithmetic operations (also referred to as calculation encryption or software encryption) can be applied to access systems and the like as described, for example, in Patent Document 1. That is, in a PON (Passive Optical Network) configuration in which an optical signal transmitted from one optical transmitter is branched by an optical coupler and then distributed individually to optical receivers of a plurality of optical subscribers' homes, signals directed to another subscriber other than a desired optical signal are inputted to each optical receiver. Thus, information data for each subscriber is encrypted using mutually different key information, so that mutual leakage and wiretapping of the information are avoided, so that security data communication is realized.
[Patent Document 1] Japanese Laid-Open Patent Publication No. H9-205420